Gimbal expansion joints (also commonly referred to as ‘gimbals’) are typically found in ducting where it is beneficial to allow relative movement between portions of the ducting. In particular, gimbal expansion joints allow rigid sections of ducting to angulate relative to one another. In, for example, a gas turbine engine bleed system, this may allow accommodation of thermal displacements and relative displacement due to airframe and engine loads. Such accommodation can reduce the stresses experienced by the system, and may also reduce the loads transferred to the airframe and engine interfaces.
A conventional gimbal expansion joint described in U.S. Pat. No. 3,232,646 is shown in FIG. 1. Two pieces of pipe ducting are interconnected by a flexible coupling which comprises two clevis-type end parts 101 and 103 connected together by an annular gimbal ring 105, having two pairs or sets of pivot members 107a, b. The members of each pair are in a diametrically opposed relationship, each pair having its pivotal axis disposed at right angles to the pivotal axis of the other pair and being pivotably connected to a respective one of the end parts 101, 103. In this way, the end parts and gimbal ring can act as a universal joint. To prevent fluid passing through the ducting leaking out of the coupling, a flexible bellows 109 is arranged externally around the gimbal ring 105, opposite ends of the bellows 109 being welded to the end parts in a fluid tight manner.
FIG. 2 shows a schematic half longitudinal cross-section through a further conventional gimbal expansion joint located on axis Y-Y. This gimbal expansion joint also has a first clevis 125 for securing to one end of a first piece of ducting, a second clevis 127 for securing to an end of a second piece of ducting, and a gimbal ring 131 to which the first and second clevises are pivotably connected at respective orthogonally disposed pairs of pivots to form a universal joint between the ends of the pieces of ducting. Each pivot in this joint, however, is formed by a pin 133 which projects outwardly from the gimbal ring to pass through a hole in the respective clevis so that the clevis can rotate about the pin. The joint has a flexible bellows 129 welded to both the clevises such that, in use, the first and second pieces of ducting are fluidly connected, but in this joint the bellows is arranged internally of the gimbal ring.
As shown in both FIG. 1 and FIG. 2, in conventional gimbal expansion joints, each weld interface between the bellows and one of the clevises is formed across curved, radially-directed faces of the bellows and the clevis. Due to tolerances on curvature, these faces may not be perfectly matched. However, by welding the interface, leakage paths between the bellows and clevises can be prevented from forming.
Other approaches for joining the curved interfaces, such as use of adhesives and fillers, are also possible, but when operating temperatures are in excesses of 150° C. (as is typically the case in the context of gas turbine engine bleed systems), welding is generally used because at such temperatures, adhesives and fillers may melt or otherwise degrade. However, welding of dissimilar materials is more problematic than welding of similar or identical materials. Accordingly, the material of the bellows and the clevises are typically selected to be formed of the same material, to facilitate their welding together. Each clevis and associated ducting may also be welded together and thus formed of the same material for the same reason.
However, this can have the effect that the material of the ducting, which is typically over 95% of the total weight of a ducting assembly including the gimbal expansion joint, may be defined by a requirement for the bellows material, which is typically under 1% of the weight of the assembly.
For example, ducting operating at relatively low temperatures (˜150° C.) that could be made from, e.g. a lightweight titanium alloy, may instead be manufactured from heavier Inconel™, solely because the bellows needs to be manufactured from Inconel™ to achieve satisfactory strength to accommodate the required flexing of the bellows.